This invention relates to the area of the isomerization processes of aromatic compounds with eight carbon atoms.
According to the known processes for isomerization of aromatic compounds with eight carbon atoms, a feedstock that is generally low in paraxylene relative to the thermodynamic equilibrium of the mixture (i.e., whose paraxylene content is clearly less than that of the mixture with the thermodynamic equilibrium at the temperature in question, whereby this mixture comprises at least one compound that is selected from the group that is formed by metaxylene, orthoxylene, paraxylene and ethylbenzene) and generally rich in ethylbenzene relative to this same mixture in thermodynamic equilibrium is introduced into a reactor that contains at least one catalyst under suitable temperature and pressure conditions to obtain a composition, at the outlet of said reactor, of aromatic compounds with eight carbon atoms that is as close as possible to the composition of said mixture in thermodynamic equilibrium at the temperature of the reactor.
Paraxylene and optionally orthoxylene, which are the desired isomers because they exhibit an important advantage particularly for the synthetic fiber industry, are then separated from this mixture. Metaxylene and ethylbenzene can then be recycled to the inlet of the isomerization reactor so as to increase the production of paraxylene and orthoxylene. When it is not desired to recover orthoxylene, the latter is recycled with metaxylene and ethylbenzene.
The isomerization reactions of the aromatic compounds with eight carbon atoms per molecule pose, however, several problems that are produced by secondary reactions. Thus, in addition to the main isomerization reaction, hydrogenation reactions are observed, such as, for example, the hydrogenation of the aromatic compounds of naphthenes, reactions of opening naphthene cycles that lead to the formation of paraffins that have at most the same number of carbon atoms per molecule as the naphthenes from which they are obtained. Cracking reactions are also observed, such as, for example, the cracking of paraffins that lead to the formation of light paraffins that typically have from three to five carbon atoms per molecule, dismutation and transalkylation reactions that lead to the production of benzene, toluene, aromatic compounds with nine carbon atoms per molecule (trimethylbenzenes, for example) and heavier aromatic compounds.
All of these secondary reactions are greatly detrimental to the yields of desired products.
The amount of secondary products that are formed (naphthenes that typically contain from five to eight carbon atoms, paraffins that typically contain from three to eight carbon atoms, benzene, toluene, aromatic compounds with, for the most part, nine and ten carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerization reactor (temperature, partial hydrogen and hydrocarbon pressures, feedstock flow rate).
It is well known to one skilled in the art that in certain catalytic processes, procedures for activating and/or selecting the catalyst are necessary to optimize the performances of the catalyst. For example, in the case of catalyst that contains a metal of group VIII of the periodic table (Handbook of Physics and Chemistry, 45th Edition 1964-65), such as, for example, platinum, it is well known to pretreat the catalyst with hydrogen sulfide (H2S). The sulfur that is contained in the hydrogen sulfide molecule is attached to the metal and imparts to it improved catalytic properties.
In addition, it has been shown that the secondary reactions increase when the paraxylene content in the reactor is closer to the paraxylene content in thermodynamic equilibrium under given pressure and temperature conditions.
The optimization of the operating conditions and the formulation of the isomerization catalyst make it possible to improve the paraxylene yield but not to be loss-free.
The invention relates to a process for isomerization of a feedstock that contains aromatic compounds with eight carbon atoms that comprises at least one isomerization stage a) that is carried out in the presence of activated catalyst according to the particular procedure that is described below and at least one dehydrogenation stage b). The process for activation of the isomerization catalysts comprises at least one sulfurization stage and at least one stage for passivation with ammonia.
It has actually been discovered that, on the one hand, catalytic performance levels are improved when a catalyst is used in a presulfurized form or a sulfurized form after introduction into the reactor and that it is subjected to a passivation in the presence of ammonia (NH3) or a precursor of ammonia and that, on the other hand, it is possible to reach paraxylene contents that are close to the paraxylene content in thermodynamic equilibrium under given pressure and temperature conditions while reducing the xylene losses by combining at least two reaction stages.
According to a particular embodiment of this invention, the feedstock that is treated in the isomerization stage contains at least ethylbenzene or at least metaxylene or at least a mixture of ethylbenzene and metaxylene.
Isomerization stage a) of the process according to the invention uses an activated catalyst which, starting from a mixture that contains aromatic compounds with eight carbon atoms including xylenes and/or ethylbenzene, makes it possible to obtain a compositionxe2x80x94xylenes and ethylbenzenexe2x80x94that is close to that of the composition of the mixture in thermodynamic equilibrium under given temperature and pressure conditions.
The activation process of said catalyst pertains to all of the catalysts for isomerization of aromatic compounds with eight carbon atoms that contain at least one metal or metal compound of group VIII that is selected from among iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, and preferably at least one noble metal or noble metal compound of group VIII, preferably selected from among platinum and palladium. This catalyst also comprises at least one matrix and optionally at least one additional element that is a metal or a metal compound that is selected from the complex that is formed by the metals or metal compounds of groups III.A and IV.A.
The catalyst that is used in stage a) of the process according to the invention is a supported catalyst and can contain at least one zeolite that is preferably selected from among the zeolites of mordenite structural type (MOR), MFI, EUO or mazzite, such as, for example, the omega zeolite.
In a preferred form of the invention, the zeolite is of MOR or EUO structural type, such as, for example, the EU-1 zeolite.
The EUO- or MOR-type zeolite contains silicon and at least one element T that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum or boron. In the case of the zeolite of EUO structural type, the overall atomic Si/T ratio is greater than 5, preferably about 5 to 100. For the zeolite of MOR structural type, the Si/T ratio is usually less than 20, and most often between 5 and 15.
The zeolite of EUO or MOR structural type according to a preferred embodiment of the invention is at least in part, preferably virtually totally, in acid form, i.e., in hydrogen form (H+), whereby the sodium content is such that the Na/T atomic ratio is less than 0.5, preferably less than 0.1.
When the catalyst contains a zeolite, said zeolite represents 1 to 90% by weight, preferably 3 to 60% by weight, and even more preferably 4 to 40% by weight relative to the total weight of the catalyst. The content by weight of said element(s) of group VIII is generally from about 0.01 to 2.0% relative to the total weight of the catalyst, preferably from about 0.05 to 1.0% relative to the total weight of the catalyst. This element of group VIII is preferably selected from the group that is formed by platinum and palladium. Most often, this element is platinum.
The catalyst of stage a) of the process of this invention optionally contains at least one additional element that is selected from the complex that is formed by elements of groups III.A and IN A VACUUM.A of the periodic table, preferably selected from the group that is formed by tin and indium. The content by weight of said element(s) is generally from about 0.01 to 2.0% relative to the total weight of the catalyst, preferably from about 0.05 to 1.0% relative to the total weight of the catalyst.
A matrix (or binder) usually ensures the addition to 100% by weight in the catalyst. It is generally selected from the group that is formed by the natural clays (for example kaolin or bentonite), synthetic clays, magnesia, aluminas, silicas, silica-aluminas, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates, preferably from among the elements of the group that is formed by the aluminas and the clays. This matrix can be a single compound or a mixture of at least two of these compounds.
The process for activation of the catalysts that are used in stage a) that can isomerize aromatic compounds that contain eight carbon atoms comprises at least one sulfurization stage and at least one stage for passivation with ammonia that are carried out in any order, whereby the sulfurization stage is generally preceded by a stage of reduction of the metal compound that is contained in the catalyst.
The sulfurization of the catalyst is carried out with a sulfur compound, for example hydrogen sulfide or a hydrogen sulfide precursor. The sulfurization of the catalyst can be carried out before introducing said catalyst into the reactor; the catalyst is then called a xe2x80x9cpresulfurized catalyst.xe2x80x9d It can also be carried out on a catalyst that is already in place in the reactor.
In general, before sulfurization, the metal compound of group VIII that is contained in the catalyst is reduced. This presulfurization stage can be carried out by pure hydrogen sulfide or by a preferably organic precursor of hydrogen sulfide, which will then be decomposed in the reactor.
Without this list having a limiting nature, the sulfurized organic compounds that can be used in the sulfurization stage are, for example, the sulfurized alkyl compounds, the sulfurized aryl compounds, and the sulfurized alkylaryl compounds. As examples, butylethyl sulfide, diallyl sulfide, dibutyl sulfide, dipropyl sulfide, dimethyl disulfide (DMDS), thiophene, dimethyl thiophene and ethylthiophene will be cited.
The sulfurization stage of the catalyst is usually carried out in a neutral or reducing atmosphere at a temperature of about 20 to 500xc2x0 C. and preferably about 60 to 400xc2x0 C., at an absolute pressure of about 0.1 to 5 MPa and preferably about 0.3 to 3 MPa and with a gas volume (inert or reducing) per volume of catalyst per hour (V.V.H.) of about 50 hxe2x88x921 to 600 hxe2x88x921 and preferably about 100 to 200 hxe2x88x921. Most often, the inert gas that is used is nitrogen, and the reducing gas is usually most often essentially pure hydrogen.
The sulfurization stage is associated with a passivation stage in the presence of ammonia (NH3). The passivation can be carried out before or after the sulfurization stage. In a preferred way, the sulfurization stage is carried out before the passivation stage. These two stages of sulfurization and passivation can be carried out before or after the introduction of the catalyst in the reactor. In a preferred way, the passivation stage in the presence of ammonia is carried out whereas the catalyst is already in place in the reactor.
The passivation with ammonia is carried out most often in two periods: at least one injection of at least a specified amount of ammonia, in NH3 vapor form, or in the form of a precursor compound of ammonia, then at least one continuous injection of ammonia in NH3 vapor form or in the form of at least one precursor compound of ammonia during the introduction of the feedstock that is to be isomerized. The duration of the injection of the second ammonia period in NH3 vapor form of this ammonia precursor depends on the duration of operation of the catalyst; in particular it depends on the stabilization of temperatures within the catalyst. The first injection is preferably carried out with NH3 in vapor form, and the second injection is carried out with at least one precursor compound of ammonia.
The precursors of ammonia (NH3) that can be used within the scope of this invention are all the compounds that are known to one skilled in the art that, in the presence of hydrogen, decompose into ammonia that attaches to the catalyst. Among the compounds that can be used, it is possible to cite the aliphatic amines, such as, for example, n-butylamine.
According to a preferred embodiment of this invention, the stages of sulfurization and passivation with ammonia are carried out after the catalyst is charged in the reactor, and the sulfurization stage is preceded by a catalyst reduction stage.
The reduction of the catalyst is carried out in the presence of hydrogen that preferably has a purity that is greater than 90 mol %. The reduction temperature is about 300 to 550xc2x0 C. and preferably about 400 to 520xc2x0 C. The total pressure is between atmospheric pressure and 3 MPa, and preferably it is from about 0.5 to 2 MPa. The duration of the reduction stage is usually from about 1 to 40 hours and preferably from about 1 to 8 hours.
The hydrogen flow rate (addition of fresh hydrogen and recycled hydrogen from the outlet to the inlet of the reactor) is from about 0.1 1/h/g to 100 l/h/g of catalyst.
When the sulfurization stage in the presence of hydrogen of the catalyst is carried out most often by using hydrogen sulfide (H2S) as a sulfurizing agent, an amount of hydrogen sulfide that corresponds to a content by weight of about 0.01 to 0.8% and preferably from about 0.01 to 0.5% relative to the mass of the catalyst is introduced into the reactor. The temperature, pressure and hydrogen flow rate conditions are identical to those of the reduction stage, in contrast, the hydrogen that is introduced into the reactor is preferably only recycled hydrogen.
The passivation with ammonia during the first period of this passivation is carried out by using gaseous ammonia or a precursor compound of ammonia, in general mixed with hydrogen.
The amount of ammonia that is introduced into the reactor is from about 0.02 to 5% by mass and preferably from about 0.1 to 2% by mass relative to the mass of the catalyst.
The temperature, pressure and hydrogen flow rate conditions are identical to those of the reduction stage; in contrast, the hydrogen that is introduced into the reactor is preferably only recycled hydrogen.
The isomerization process of a feedstock that contains aromatic compounds with eight carbon atoms comprises at least one isomerization stage a) that is carried out in the presence of an activated catalyst according to the preceding activation process and that contains at least one metal of group VIII and preferably at least one zeolite of EUO or MOR structural type, at least one matrix and optionally at least one additional element and at least one dehydrogenation stage b).
In the first stage of the isomerization process according to this invention, the operating conditions of the isomerization zone are selected to reduce the production of undesirable compounds that are obtained from reactions that cause acid catalysis mechanisms (cracking, dealkylation, dismutation, . . . ) to take effect. These operating conditions are such that the production of naphthenes with eight carbon atoms per molecule is significantly largerxe2x80x94about 10 to 30% by mass of the outlet effluent of the isomerization zonexe2x80x94than the production that is obtained by standard isomerization processes of aromatic compounds that contain eight carbon atomsxe2x80x94which is generally from about 5 to 10% by mass of the outlet effluent of the isomerization zone.
The effluent that is obtained at the end of the first reaction stage is treated during a second stage in a reaction zone that contains at least one dehydrogenation catalyst. The operating conditions of this second stage can be different from or identical to the operating conditions of the first stage, preferably the operating conditions of these two stages are different. The operating conditions of this second stage are determined so as to obtain a composition of the mixture of xylenes and ethylbenzene that is the closest possible to the composition in thermodynamic equilibrium.
The catalysts for dehydrogenation of paraffins and naphthenes are well known to one skilled in the art. The substrates of these catalysts are generally refractory oxides; most often an alumina is selected. These dehydrogenation catalysts comprise at least one noble metal of group VIII of the periodic table and at least one alkaline element or alkaline earth element of groups I.A and II.A of the periodic table. Preferably, the noble metal of group VIII that is selected is platinum, and the element of groups I.A or II.A of the periodic table is selected from the group that comprises magnesium, potassium, and calcium.
These dehydrogenation catalysts can also contain thorium and/or at least one element M of groups IV.A or IV.B of the periodic table. The elements of groups IV.A or IV.B are most often selected from the group that is formed by tin, silicon, titanium and zirconium. Some dehydrogenation catalysts also contain sulfur and/or a halogen. More particularly, it is possible to use the dehydrogenation catalysts that are described in U.S. Pat. Nos. 3,998,900 and 3,531,543 in the dehydrogenation stage of the process according to this invention.
Without wanting to be tied to any particular theory, it is noted that platinum exhibits a hydrogenolyzing activity that is expressed to the detriment of the activity of the dehydrogenation of naphthenes into aromatic compounds. This hydrogenolyzing activity can be greatly reduced, and the selectivity of the catalyst relative to the dehydrogenation reaction can be increased by adding additional element M.
The refractory inorganic substrates that are used often have an acidic nature and can generate undesirable secondary reactions, such as cracking or isomerization reactions. This is why the oxide substrate is generally neutralized by the addition of at least one metal or an alkaline metal compound or an alkaline-earth metal compound.
According to a preferred embodiment of this invention, at least one compound that has a boiling point of about 80 to about 135xc2x0 C., preferably at least one compound that is selected from the group that is formed by the paraffins with eight carbon atoms per molecule, benzene, toluene, and naphthenes with eight carbon atoms, is added to the feedstock that is introduced in the isomerization zone.
This compound or these compounds are added to the feedstock that is to be treated in the form of recycling and/or in the form of fresh compounds in amounts such that the percentages per unit of mass of added compounds relative to the total feedstock that enters the reactor are usually as follows:
the percentage of paraffins with eight carbon atoms, in the optional case where this compound is added, is from about 0.1 to 10% by mass, preferably from about 0.2 to 2% by mass,
the percentage of naphthenes with eight carbon atoms, in the optional case where this compound is added, is from about 0.5 to 15% by mass, and preferably from about 2 to 8% by mass,
the percentage of toluene, in the optional case where this compound is added, is from about 0.1 to 10% by mass, preferably from about 0.2 to 5% by mass,
the percentage of benzene, in the optional case where this compound is added, is from about 0.1 to 10% by mass, preferably from about 0.2 to 2% by mass.
The percentage of total compounds that are added when several compounds are added represents about 0.1 to 20% by mass and often about 2 to 15% by mass relative to the total feedstock that enters the isomerization zone.
According to a preferred embodiment of the invention, at least two different compounds that each have a boiling point of about 80xc2x0 C. to 135xc2x0 C. are introduced into the reaction zone. More particularly, at least one naphthene with eight carbon atoms and at least one paraffin with eight carbon atoms are introduced. In another variant, when these compounds are obtained from recycling of a liquid fraction that leaves the dehydrogenation reactor, all of the compounds that are contained in this liquid fraction that have boiling points of about 80xc2x0 C. to 135xc2x0 C. are introduced without being separated.
In the process of this invention, the isomerization stage is used in the presence of hydrogen that can be introduced in the form of fresh hydrogen, in the form of recycled hydrogen that is obtained from the outlet of the isomerization zone or in the form of recycled hydrogen that is obtained from the outlet of the dehydrogenation zone. The operating conditions of the isomerization stage are as follows: a reaction temperature of about 300 to 500xc2x0 C., preferably of about 320 to 400xc2x0 C., a partial hydrogen pressure of about 0.3 to 1.5 MPa, preferably of about 0.4 to 1.2 MPa, a total pressure of about 0.4 to 2 MPa, preferably of about 0.6 to 1.5 MPa, and a P.P.H. (feedstock weight/catalyst weight/hour) of about 0.2 to 10 hxe2x88x921, preferably of about 3 to 6 hxe2x88x921.
In the process according to this invention, the dehydrogenation stage is used in the presence of hydrogen that can be introduced in the form of fresh hydrogen, in the form of recycled hydrogen that is obtained from the outlet of the isomerization zone or in the form of recycled hydrogen that is obtained from the outlet of the dehydrogenation zone.
The operating conditions for the dehydrogenation stage are a temperature of about 300 to 500xc2x0 C., preferably of about 400 to 420xc2x0 C., a partial absolute hydrogen pressure of about 0.1 to 1.5 MPa, preferably of about 0.4 to 1 MPa, a total absolute pressure of about 0.2 to 2 MPa, preferably of about 0.5 to 1.5 MPa and a PPH (feedstock weight/catalyst weight/hour) of about 0.2 to 10 hxe2x88x921, preferably of about 3 to 6 hxe2x88x921.
In addition, it is also possible to carry out a recycling of aromatic compounds with eight carbon atoms that are contained in the effluent of the dehydrogenation zone after the desired compounds, i.e., paraxylene and optionally orthoxylene, have been extracted.